DAHP (3-deoxy-D-arabino-heptulosonic acid 7-phosphate) synthease EC 4.1.2. 15, the first enzyme of the multibranched pathway that leads to the synthesis of the three aromatic amino acids phenylalanine, tryosine, and tryptophan and to the synthesis of several vitamins and the iron-chelating compound enterochelin, appears in Escherichia coli in three isoenzymic forms, each feedback-inhibited by one of the aromatic amino acids. We have purified to homogeneity two of the isoenzymes, and we have started the physico-chemical characterization of these proteins. Amino acid sequence analysis of the tyrosine-sensitive isoenzyme will be started, and the crystallization of this protein will be attempted. The isolation of a multiple copy plasmid carrying the structure gene for the phenylalanine-sensitive DAHP synthase will be trided. DNA heteroduplex analysis of phage lambda DNA's containing the structural genes for the tyrosine- or the phenylalanine-sensitive isoenzyme will be undertaken. Monospecific antibodies against the tryosine-sensitive DAHP synthase will be prepare and used as tools for in vitro biosynthesis of this enzyme.